Humble bolts seems like a mundane and boring machine component. However, we engineers always get a little excited at threaded fasteners, simply for the incredible variety available and their significant capability in any engineering application. You see them in bridges and buildings, aircraft, cars and of course, our motorcycles.
Why would you choose to use threaded fastener? Usually, for flexibility in assembling or disassembling components. Sometimes the cost of using a threaded fastener to secure a joint might be cheaper than other options (such as welding a joint together). In our motorcycles, there will be a combination of reasons.
How do bolts work?
A threaded fastener (lets just simplify and say “bolts” from now on, it will save me typing “threaded fastener” another 100 times) works like a spring. Remember our discussion in the Basic properties of metals article about the tensile test. A bolt works in tension (meaning the load is parallel to its long centreline axis). When you put a bolt through, say, two pieces of steel, and tighten the nut, the steel plates do not compress appreciably, so the distance between the nut and the bolt head is essentially fixed. However if the bolt is being tensioned, it is stretching between the nut and the bolt head, which are up tight against our theoretical steel plates. If the bolt is stretching, then it is exerting an equal but opposite clamping force on the two steel plates. This diagram might help illustrate the point.
Image courtesy of http://www.longlok.com/Self-Locking-and-Self-Sealing-Fasteners
It is this clamping force which is important. The clamping force is intended to prevent any slip between the parts. The amount of operational forces the engineer expects the assembly to withstand in service, will, in turn, dictate how much clamping force is required between the parts. This clamping force will then inform the bolted joint design. How many bolts can I fit on this assembly? What size bolts can I fit here? Are there particular geometry features that I have to consider which limits my bolt choice? Do I have to change the shape of the part to fit enough bolts into the assembly?
Over the industrial age through to modern times, various thread forms have been developed by different industries in different countries. This is what they look like in cross section:
Image courtesy of http://www.me.metu.edu.tr/courses/me114/Lectures/screw_threads.htm
The obsessive-compulsive amongst us can dig further into this topic, but the for the purpose of our motorcycles, we are really only interested in the last 50 years or so, which simplifies our review of thread forms.
Common thread forms we might encounter on our bikes will likely be limited to the following three:
- British Standard Whitworth and British Standard Fine in inch sizes, usually found on older British bikes,
- Unified National (Coarse) and Unified National (Fine) in inch sizes, common on American bikes,
- Metric (coarse and fine) in millimetre sizes, found on Japanese and more modern (1970’s onwards) European bikes
A fine thread is useful when you are threading into relatively thin material. A coarse thread may only have a couple of turns into a thin piece. When it is loaded in operation there is a chance the threads will strip. A fine thread means more are engaged in the same thickness material, which means more surface area, which means less internal pressure, and this reduces the chance of thread damage.
So, how do you identify the thread on the bolt you have just removed from your bike?
You could watch this video:
Or, use a thread pitch gauge. This tool has a bunch of “leaves”, each with a specific “saw-tooth” pattern to match a particular thread.
To use the gauge, you need to take an overall measurement of the diameter over the threads, then match one of the gauge leaves to the thread pitch on the bolt. Threads are all standardised now so you’ll have to find your match from a table (in other words, the choices are NOT infinite!).
Clearly, if we are trying to identify Whitworth, Unified National or Metric threads, we need three different gauges. This can get expensive. A more economical method is to purchase The Fastener Black Book from a local vendor and make use of the hole and thread identification gauge that is supplied.
First match the bolt diameter to one of the holes in the gauge:
Then, match the thread pitch with the bolt you are using to one of the thread patterns around the perimeter of the gauge:
If you are trying to identify the thread form of a nut, you really need to measure the matching bolt to make this process bulletproof (and a lot easier).
If for example, you are replacing a damaged bolt, you need to be able to specify the basic diameter, the thread form, and the length.
Metric bolts are designated with a capital “M” before the basic diameter. So, M12, is a metric 12mm bolt, where 12mm is the diameter of the bolt shank. The thread form is then designated by the “pitch”, or in simple terms, the distance between the threads. So “M12 x 1.75” is a metric 12mm bolt with a 1.75mm pitch. Finally, we need the length. Sometimes this might be added to the end of the designation – “M12 x 1.75 x 50”, or, sometimes on technical drawings it will be shown “M12 x 1.75 50LG” where “50LG” means “50mm long”.
Unified National bolts are a little different. The basic diameter is, again, the bolt shank and is generally given in fractions of an inch – 1/4″, 5/16″, 3/8″, 1/2″ and so on. The thread form is specified by “threads per inch” which is abbreviated “TPI”. This allows you to distinguish between coarse and fine threads. Sometimes you’ll simply see them designated as either “UNC” (coarse) or “UNF” (fine), such as “3/8 UNC”, other times as “3/8-16″ (3/8” diameter, 16 threads per inch) for the coarse version, or “3/8-24″ (3/8” diameter, 24 threads per inch) for the fine version.
If you are unlucky enough to have to worry about Whitworth threads, then read on. Whitworth was the world’s first standardised thread form, and the term includes the fine thread version. They are simply designated by the basic diameter of the bolt shank. “BSW” (British Standard Whitworth) designates the “standard” thread, and “BSF” (British Standard Fine) designates the find thread.
If you don’t have any reference books, simply Google “thread tables” and check out the images for different thread tables.
You really need some definite way of identifying bolts. You could try and match them to an existing known nut, but this can be hit and miss. I strongly recommend some sort of thread gauge. I don’t get any kickbacks from The Fastener Blackbook but I highly recommend it (or something similar). If you are on a journey to customise a bike, some of your purchases are investments in your skills and knowledge, as much as parts for your bike.
We’ll talk about bolt materials, bolt strength, drilling holes, tapping and repairing threads in a future article. This Tech Note really is the basis for those future articles.
Please contact me if you have any comments, thoughts or feedback on this article.